This invention relates to metallic hollow metallic nanostructures and microstructures. Microstructures and nanostructures having hollow interiors, for example, nanoshells, have many potential applications because of their increased surface areas, low density, low material cost, and sometimes special optical properties. Most previous nanoshell and microshell structures have been synthesized using multiple bottom-up processes by coating a hard core template followed by removing the core template through etching. For example, Kim et al. have synthesized palladium hollow spheres using silica spheres as a template (S.-W. Kim, M. Kim, W. Y. Lee, and T. Hyeon, “Fabrication of Hollow Palladium spheres and Their Successful Applications to the Recyclable Heterogeneous Catalyst for Suzuki Coupling Reactions,” J. Amer. Chem. Soc. Vol. 124 (2002) pp. 7642-7643). The surfaces of the silica spheres were functionalized with mercaptopropylsilyl (MPS) groups. The palladium precursor, palladium acetylacetonate, was then adsorbed onto the surfaces of the MPS-functionalized silica spheres. The Pd2+-adsorbed-MPS-functionalized silica spheres were heated at 250 C for 3 hours to obtain Pd metal-coated spheres. The silica sphere was removed by HF etching.
Hollow nanoshells of gold have been prepared by leaching out silver chloride from Aushell/(AgCl+horseradish peroxidase)core nanoparticles with dilute ammonia solution (R. Kumar, A. N. Maitra, P. K. Patanjali, and P. Sharma, “Hollow gold nanoparticles encapsulating horseradish peroxidase,” Biomaterials, vol. 26 (2005), pp. 6743-6753).
Galvanic replacement processes where a sacrificial metal template is used have also been reported. Reaction of Pd (NO2)2 or Pt(CH3COO)2 with Ag nanocrystal templates has produced Pd and Pt nanoshells (Y. Sun, B. T. Mayers, and Y. Xia, “Template-Engaged Replacement Reaction: A One-Step Approach to the Large-Scale synthesis of Metal Nanostructures with Hollow Interiors,” Nano Letters vol. 2 (002) pp. 481-485). The shape of the nanocrystal template is reproduced in the nanoshell structure. Liang et al. have synthesized Pt hollow nanospheres by exploiting the replacement reaction between Co nanoparticles and H2PtCl6 (H.-P. Liang, H.-H. Zhang, J.-S. Hu, Y.-G. Guo, L.-J Wan, and C.-L. Bai, “Pt Hollow Nanospheres: Facile Synthesis and Enhanced Electrocatalysts,” Angew. Chem. Int. Ed. Vol. 43 (2004) pp. 1540-1543). Co nanoparticles are oxidized to cobalt ions when the solution of Co nanoparticles is added to a H2PtI6 solution. The reaction continues until the Co is completely consumed. The Pt shell is incomplete and porous.
Emulsion droplets have been used as coating templates for synthesis of hollow spheres of oxides and semiconductors. These previous emulsion-based methods did not employ photocatalytic interfacial synthesis of metallic nanoshells or microshells using a porphyrin-stabilized emulsion template.
Schacht et al. have combined long-range oil-in-water emulsion and oil-in-water interface physics with the shorter range cooperative assembly of silica and surfactants at the oil-water interface to create ordered composite mesostructured phases that are also macroscopically structured (S. Sachact, Q. Huo, I. G. Voign-Martin, G. D. Stucky, and F. Schuth, “Oil-Water Interface Templating of Mesoporous Macroscale Structures,” Science, Vol. 272, (1996) pp. 768-771).
A related patent application that does not employ emulsions to form shells is John A. Shelnutt, Yujiang Song, Eulalia F. Pereira, and Craig J. Medforth, “Dendritic Metal Nanostructures,” U.S. patent application Ser. No. 10/887,535 filed Jul. 8, 2004. It is incorporated herein by reference.